Carbon black is widely used as a colorant in water-based inks for printing a black color with ink jet printers.
In general a carbon black raw material powder comprises a secondary agglomerated mass called an agglomerate formed through van der Waals attraction or mere congregation, adhesion or interlocking between aggregates that are primary aggregated bodies in each of which approximately spherical primary particles are congregated together. Unless subjected to treatment, this type of carbon black raw material powder immediately settles without being dispersed in water.
A water-based ink containing such a raw material powder and a dispersant for dispersing the raw material powder has high viscosity, and hence printing is difficult. Modified carbon blacks obtained by oxidizing the surface of the carbon black raw material powder to introduce hydrophilic functional groups such as carboxyl groups so that dispersion in water is easy even if a dispersant is not used are thus used.
As such a modified carbon black, in Patent Document 1 the present inventors have disclosed one having a specified specific surface area and DBP oil absorption (the amount of oil absorption using dibutyl phthalate; referred to as the ‘DBP oil absorption’ in the present specification). Water-based inks containing this modified carbon black have a low viscosity, and moreover have a high reflection density (optical density; referred to as the ‘OD value’ in the present specification), and hence dense printing with a deep black color is possible. When using a water-based ink as an ink jet recording ink, the water-based ink must have sufficient stability over time for practical use, but the above water-based inks have not always had sufficient stability over time.
Moreover, there are modified carbon blacks obtained by oxidizing a carbon black raw material powder having a small mean particle diameter. A water-based ink containing such a modified carbon black has good dispersibility with settling not being prone to occur, and hence the storage stability is excellent, and clogging is not prone to occurring. However, if printing is carried out using such an ink, then the reflection density is low, and a printing density sufficient for practical use is no longer exhibited. Moreover, the hue of the ink tends to become a reddish black.
Moreover, in Patent Document 2 there is disclosed a carbon black pigment for water-based inks for which N2SA, IA, N2SA/IA, CTAB, DBP, 24M4 DBP, Tint, and the ratio of total oxygen atoms to total carbon atoms are specified. However, this carbon black pigment for water-based inks has a problem that the OD value is not sufficient in the case of use as an ink jet ink in particular.    (Patent Document 1) Japanese Patent Application Laid-open No. 2000-319573    (Patent Document 2) Japanese Patent Application Laid-open No. 2001-81355
The present invention was accomplished to resolve the above problems; it is an object of the present invention to provide a carbon black that can be used in a water-based ink that is used in ink jet printer printing, that has a reduced settling rate and can thus be stored stably for a long period, for which clogging is not prone to occurring, and for which the reflection density (OD value) is high and hence dense printing with a deep black color is possible, and a water-based ink having this carbon black dispersed therein.